//
// Created by Jisam on 2024/8/4 20:07.
// solve 清楚姐姐买竹鼠
#include <bits/stdc++.h>

#define PSI pair<string,int>
#define PII pair<int,int>
#define PDI pair<double,int>
#define PDD pair<double,double>
#define VVI vector<vector<int>>
#define VI vector<int>
#define VS vector<string>

#define PQLI priority_queue<int, vector<int>, less<int>>
#define PQGI priority_queue<int, vector<int>, greater<int>>
using u32 = unsigned;
using i64 = long long;
using u64 = unsigned long long;

using namespace std;


void solve() {
    long long a = 1e9+1, b=1e9*4, c= 1;
    cin >> a >> b >> c;
    cout << min({c / 3 * b + c % 3 * a,a * c,(c + 2) / 3 * b});
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int T = 1;
//    cin >> T;
    while (T--) solve();
//    cout << "\n"[];
    return 0;
}